1. Field of the Invention
The present invention relates to a method for the electrolysis of an alkali metal chloride using a cation exchange membrane More particularly, the present invention is concerned with a method for the electrolysis of an alkali metal chloride in an electrolytic cell, in which use is made of a cation exchange membrane comprising a carboxylate-containing fluorocarbon polymer layer disposed on a cathode compartment side and provided on its surface with an inorganic coating, and a sulfonate and/or carboxylate-containing fluorocarbon polymer layer disposed on an anode compartment side and having a specific water content. By this method, an alkali metal hydroxide, e.g., sodium hydroxide, can be directly produced in a concentration as high as 45-55% by weight from a saline solution having an alkali metal chloride concentration as low as from 3.0 to 3.5N with reduced energy consumption, stably for a prolonged period of time.
2. Discussion of Related Art
The process for electrolyzing an alkali metal chloride into an alkali metal hydroxide and chlorine using an ion exchange membrane of a fluorocarbon polymer is now widely used, because a high quality alkali metal hydroxide can be produced with low energy consumption. Especially, in the art of sodium chloride electrolysis using an ion exchange membrane, a cation exchange membrane of a fluorocarbon polymer having carboxylate groups on its cathode compartment side is advantageously used, because an extremely high current efficiency is stably obtained for a prolonged period of time.
The sodium hydroxide (hereinafter frequently referred to as "caustic soda") is now generally distributed on the market in the form of an aqueous solution having a concentration of about 50% by weight, whereas the caustic soda solution produced by electrolysis now generally has a concentration of only up to about 40% by weight. Accordingly, in the art, the problem of this difference in concentration has been coped with by concentrating the caustic soda solution by means of steam or other energy source. Therefore, there has been a strong demand in the art for a method by which caustic soda can be directly produced in a concentration equal to that of the commercially distributed caustic soda with high efficiency, without additional energy consumption for concentrating. However, the conventional electrolyzing methods using an ion exchange membrane have had a drawback in that although high current efficiency is obtained in the production of caustic soda having a concentration of up to about 40% by weight, the production of caustic soda of higher concentrations than about 40% by weight is likely to lower current efficiency and increase electrolytic voltage to thereby cause an increase in energy consumption.
Further, as described in U.S. Pat. No. 4,212,713, at a temperature as high as 80.degree. C. or above, the carboxylate groups of an ion exchange membrane are extremely unstable and gradually decomposed in a high concentration aqueous solution of an alkali metal hydroxide.
Intensive efforts have been made to develop an ion exchange membrane which is free from such instability in a high temperature, high concentration solution, and which is useful for efficient, direct production of a high concentration caustic soda stably for a prolonged period of time. However, none of the proposals hitherto made is satisfactory in some respects, e.g., durability of the membrane and energy consumption.
International Patent Application Publication No. WO88/09799, as a representative example of the hitherto made proposals, suggests a method in which use is made of a membrane having a porosity-regulated layer containing inorganic material particles disposed on its cathode compartment side in order to protect carboxylate groups. However, according to investigations by the present inventors, the inorganic material particles contained in the porosity-regulated layer as disclosed in the specification of this patent application are inappropriate from the viewpoint of particle size, etc., so that the adhesion of the layer to the membrane is unsatisfactory, thereby bringing about a stability problem.
U.S. Pat. No. 4,900,408, as another example of the hitherto made proposals, discloses a method in which use is made of a membrane comprising a carborylate group-containing layer having a predetermined equivalent weight and, disposed on its surface on a cathode compartment side, a porous layer capable of catholyte diffusion, and in which a saline solution having a NaCl concentration nearly equal to the saturation concentration at the temperature at which electrolysis is performed is introduced, to thereby produce sodium hydroxide in a concentration of from 42 to 54% by weight. As described in the descriptive portion and the examples of the specification of the U.S. patent, the sodium chloride concentration of the saline solution must be 3.52N to produce caustic soda in a concentration of 42% by weight, and the sodium chloride concentration of the saline solution must be 4.2N to produce caustic soda in a concentration of 54% by weight. Accordingly, in order to obtain caustic soda in a concentration of from 45 to 55% by weight as desired in the present invention, the sodium chloride concentration of the saline solution must be about 4N or above. The requirement for concentrations as high as 4N or above is disadvantageous in the commercial production of caustic soda. Further, in the method of this U.S. patent, the amount of water passing through the membrane from the anode compartment to the cathode compartment together with sodium ions, which governs the maximum concentration of produced caustic soda solution, is controlled by regulating the structural unit composition of the carboxylate-containing copolymer provided on the side of the cathode compartment. According to investigations by the present inventors, however, this method is inevitably accompanied by an elevation of electrolytic voltage, thereby increasing energy consumption.
As described above, all of the conventional methods for the production of caustic soda in a high concentration substantially equal to that of the commercially distributed caustic soda have inherent problems in that the sodium chloride concentration of a saline as a feedstock must be inevitably high, thereby causing economical disadvantages, and that energy consumption is large.